The present invention relates to a device for sensing the partial pressure of oxygen in a gas, and more particularly to an active multilayer sensor utilizing an oxygen ion conducting material. The present invention also relates to a combined sensor for measuring oxygen content and NOx content in a gas. NOx is utilized herein to represent nitric oxide, nitrogen dioxide, nitrogen trioxide, etc.
It is widely recognized that one of the most important diagnostics for monitoring the efficiency of any combustion process is the measurement of the oxygen partial pressure in an exhaust gas. Thus, oxygen sensors have long been used to measure the oxygen content of exhaust gases from such diverse combustion processes as internal combustion engines in motor vehicles and coal, natural gas, or oil burning power generation facilities.
The most widely known and used oxygen sensors are based on partially stabilized zirconia (PSZ) as the ion conductor. Such sensors function by monitoring the electromotive force (EMF) developed across an ion conductor which is exposed to different partial pressures of oxygen. Oxygen tends to move from a gas containing a high concentration of oxygen to one of lower concentration. If two gases are separated from each other by an electroded oxygen ion conductor, the oxygen molecules will dissociate on one surface of the conductor and absorb electrons to form oxygen ions. These ions then diffuse through the ionic conductor, leaving the entry surface with a deficiency of electrons (O2+4e=2Oxe2x88x922). On the exit or low oxygen concentration side of the conductor, oxygen ions leaving the conductor must give up electrons to form molecular oxygen, thus leaving the exit surface with an excess of electrons. This creates the EMF between the two surfaces of the ion conductor.
One problem with the use of partially stabilized zirconia sensors is that they must be operated at temperatures in the range of about 800 C. to reduce internal resistance to a point where a current can be measured. Further, the raw material costs of stabilized zirconia is relatively high, and the melting point of zirconia is quite high (2700 C.) so that formation of sensors is expensive.
Lawless, in U.S. Pat. No. 4,462,891, describes a passive oxygen sensor using ceramic ion conducting materials based on nickel niobates and Bismuth oxides. The oxygen sensor includes a plurality of layers of the ceramic material and a porous metallic conductor arranged to form a body having alternating ceramic and metallic layers, with first alternate ones of the metallic layers being exposed along one side of the body and second alternate ones of the metallic layers being exposed along an opposite side of the body. The first and second alternate ones of the metallic layers are exposed to separate gases, one of the gases being a reference gas, in order to create a voltage output signal across electrodes connected to alternate metallic layers. The voltage output signal is indicative of the relative oxygen partial pressures of the separate gases. Thus, the passive oxygen sensor cannot provide an oxygen partial pressure indication unless the first and second metallic layers present in the body are exposed, respectively, to a sample gas and a separate reference gas having a known oxygen partial pressure, i.e., each side of the sensor body must be exposed to a separate gas.
More recently, amperometric sensors have been introduced which also use partially stabilized zirconia but which do not require a reference gas to operate. Such a sensor 80 is illustrated in FIG. 1 and comprises a cavity 100 in communication with the unknown gas through a diffusion hole 120. The base of the cavity 100 is a PSZ electrolyte 140 which is connected through electrodes 160, 160xe2x80x2 to a voltage source 170. The application of a voltage causes oxygen to be pumped from the cavity through diffusion into the surrounding gas as shown by the arrows. If the cavity is sealed atop the base, and if the top of the cavity has the small diffusion hole 120, then a point is reached on increasing the voltage where no more oxygen can be pumped out of the cavity than is entering through the diffusion hole. The current drawn at this point is called the amperometric current. The larger the oxygen partial pressure in the surrounding gas, the larger will be the amperometric current. Thus, a measurement of the amperometric current yields the oxygen partial pressure. Again, however, this sensor suffers from some of the same drawbacks in that materials and fabrication costs are relatively high. An extremely small diffusion hole is required, about 5 xcexcm, and requires precise machining because the size is critical to the operation of the sensor. Additionally, the manufacture of the sensor of FIG. 1 requires five silk screen operations and four burnout steps. Finally, these sensors lose their sensitivity above about 80% oxygen and the diffusion hole is prone to plugging.
Accordingly, there remains a need in the art for an amperometric oxygen sensor which is relatively inexpensive to manufacture and provides enhanced oxygen sensitivity. There is also a need in the art for a sensor which is capable of providing an independent indication of NOx content in a gas.
These needs are met by the present invention wherein a combined oxygen and NOx sensor is provided. Generally, the combined sensor employs a sensor body that includes two different types of electrodesxe2x80x94oxygen-porous electrode layers and dissociative oxygen-porous electrode layers.
In accordance with one embodiment of the present invention, a combined sensor for measuring oxygen content and NOx content in a gas is provided. The sensor comprises a sensor body, an oxygen content electrical signal output, and a NOx content electrical signal output. The sensor body is disposed in the gas and comprises (i) a plurality of oxygen-porous electrode layers, (ii) a plurality of dissociative oxygen-porous electrode layers, wherein the dissociative oxygen-porous electrode layers comprise a material selected to catalyze dissociation of NOx into nitrogen and oxygen, and (iii) a plurality of oxygen ion conductive ceramic layers interposed between respective ones of the oxygen-porous electrode layers and respective ones of the dissociative oxygen-porous electrode layers. The oxygen content electrical signal output is coupled to the plurality of oxygen-porous electrode layers. Similarly, the NOx content electrical signal output is coupled to the plurality of dissociative oxygen-porous electrode layers. The NOx content electrical signal output is electrically isolated from the oxygen content electrical signal output.
In accordance with another embodiment of the present invention, a combined sensor for measuring oxygen content and NOx content in a gas is provided where the dissociative oxygen-porous electrode layers comprise sufficient Rh to catalyze dissociation of NOx into nitrogen and oxygen. In accordance with yet another embodiment of the present invention, a combined sensor for measuring oxygen content and NOx content in a gas is provided. The sensor comprises a partial enclosure defining a gas passage, a sensor body, and a diffusion barrier. The diffusion barrier defines a diffusion-limited portion of the gas passage and the sensor body is disposed in the diffusion-limited portion of the gas passage.
In accordance with yet another embodiment of the present invention, a sensor body is provided comprising a plurality of oxygen-porous electrode layers, a plurality of dissociative oxygen-porous electrode layers, and a plurality of oxygen ion conductive ceramic layers. The dissociative oxygen-porous electrode layers comprise a material selected to catalyze dissociation of NOx into nitrogen and oxygen. The plurality of oxygen ion conductive ceramic layers are interposed between respective ones of the oxygen-porous electrode layers and respective ones of the dissociative oxygen-porous electrode layers.
Accordingly, it is an object of the present invention to provide an improved oxygen and NOx sensing device. Other objects of the present invention will be apparent in light of the description of the invention embodied herein.